Ten Tables (just outside Harvard Square) can do a vegan tasting menu with ~24 hours notice (for around $30 for four courses, I think). Also, Elephant Walk (Porter Square) has a number of vegan options on the menu.

Upstairs on the Square has been fairly accommodating for a vegan friend. The upstairs restaurant has a vegan tasting menu (although I've never eaten there), and the Monday Club (which is a bit cheaper and less formal) has created vegan entrees.

Pizzera Posto in Davis Square has a vegan pizza, although only one and not many other vegan options. Pulse Cafe - which has gotten mixed reviews - is right next to Posto and is completely vegan.